System, method, apparatus and article of manufacture for identity-based caching (#15)

ABSTRACT

A process for obtaining a copy of a data object is disclosed. A location-independent identifier associated with the desired data object is obtained, for example, from a primary file that requires a copy of the data object. Using the location-independent identifier associated with the desired object, a cache is interrogated to determine whether a copy of the data object is cached. If the data object is cached, a copy of the cached data object is obtained from the cache. If the data object is not cached, a network call is performed obtain a new copy of the data object.

COPYRIGHT NOTIFICATION

Portions of this patent application contain materials that are subjectto copyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document, or the patentdisclosure, as it appears in the Patent and Trademark Office.

FIELD OF THE INVENTION

This invention relates to a system, method and apparatus for improvingresponse time of a network based transaction session. More particularly,the invention relates to a system, method and apparatus to alleviatenetwork load by use of a location-independent index cache to eliminaterepetitive and/or redundant transfers of data objects already residentin the cache.

BACKGROUND OF THE INVENTION

In a number of interactive network applications, it is common for a userto request access to data that is housed on a remote site, but whichdata has previously been transferred to the user during an earliersession, or an earlier point of time in the present session. The processof requesting the transfer of such information over the network consumessignificant time and network resources, particularly if the resourcerequested is of a substantial size. Therefore, it is desirable toprovide a means to recognize that a particular resource stored at aremote location is already available on the users local computer system,and to obtain the desired copy from that local version, rather thanperforming a network call to obtain a new identical copy. In the past,programs such as World Wide Web browsers (such as Sun Microsystems,Inc.'s HotJava, Netscape Communications Corp.'s Netscape Navigator andMicrosoft Corp.'s Internet Explorer) frequently have operated using acache indexed by a Uniform Resource Locator (URL). By means of thiscache, the WWW browser can recognize when a particular URL has beenpreviously referenced, and may present data that had been previouslyobtained in response to the previous request for the same URL.

However, one shortcoming of this approach is that a URL-indexed cache iscapable of providing the user with the previously recovered copy of therequested resource only when the second request for the resource isrequesting that resource from the exact location as that from which theresource was initially obtained. That is, if the user requests that aresource be obtained from a new location, a WWW browser equipped with aURL-indexed cache will obtain the requested resource from the specifiedlocation, even if the same data is already resident on the user'scomputer system from a previous transfer from a different location. Thisapproach results in an undesirable amount of redundant data transfers.

This problem is particularly aggravated when the resource requested isnot requested under the user's direct control. An example of such a caseis where a user requests a particular resource, and that resource inturn requires additional resources. The user has no opportunity toindicate to the system that previously obtained resources should be usedinstead of redundant copies of the secondary resources. For example, auser may request a particular web page to be displayed, and that pagemay in turn request transmission of a graphic image to be displayed, ora program (often referred to as an "applet") to be executed inconjunction with the displayed page. The user has no opportunity toindicate to the browser that a previously obtained image file or appletfile should be used in lieu of the file located at the remote location.

It is desirable, therefore, to support a remotely located resource thatresides on a plurality of computer systems that are identified by alocation-independent identifier, thereby allowing reuse of a previouslyobtained copy of that resource, even if the previously obtained copy wasobtained from a different location.

SUMMARY OF THE INVENTION

A system, method, and apparatus for obtaining a copy of a data object isdisclosed. A location-independent identifier associated with the desireddata object is obtained, for example, from a primary file that requiresa copy of the data object. A cache is interrogated to determine whethera copy of the data object is cached. If the data object is cached, acopy of the cached data object is obtained from the cache. If the dataobject is not cached, a network call is performed obtain a new copy ofthe data object.

Additional features of the invention will become apparent uponexamination of the description that follows, particularly with referenceto the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages are betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a block diagram of a representative hardware environment inaccordance with a preferred embodiment;

FIG. 2 depicts a client computer operating a network client program incommunication with a server computer;

FIG. 3 depicts an example of a downloaded HTML document;

FIG. 4 depicts an operation of obtaining an applet specified by anapplet tag;

FIG. 5 depicts the server response to an HTTP GET request;

FIG. 5A depicts the format of a Java applet class file;

FIG. 5B depicts the format of a GIF Application Extension data area;

FIG. 6 depicts transmission of a second HTML document from a second HTTPserver to the client computer over a communications link;

FIG. 7 depicts an example of a second HTML document specifying retrievalof an applet;

FIG. 8 depicts the process by which the client computer obtains a copyof the desired object from a cache;

FIG. 9 is a flow chart depicting the overall operation of the invention;and

FIG. 10 depicts an embodiment of the present invention in which a singlecache is shared by a plurality of clients.

DETAILED DESCRIPTION

A preferred embodiment of a system in accordance with the presentinvention is preferably practiced in the context of a personal computersuch as the IBM PS/2, Apple Macintosh computer or UNIX basedworkstation. A representative hardware environment is depicted in FIG.1, which illustrates a typical hardware configuration of a workstationin accordance with a preferred embodiment having a central processingunit 10, such as a microprocessor, and a number of other unitsinterconnected via a system bus 12. The workstation shown in FIG. 1includes a Random Access Memory (RAM) 14, Read Only Memory (ROM) 16, anI/O adapter 18 for connecting peripheral devices such as disk storageunits 20 to the bus 12, a user interface adapter 22 for connecting akeyboard 24, a mouse 26, a speaker 28, a microphone 32, and/or otheruser interface devices such as a touch screen (not shown) to the bus 12,communication adapter 34 for connecting the workstation to acommunication network (e.g., a data processing network) and a displayadapter 36 for connecting the bus 12 to a display device 38. Theworkstation typically has resident thereon an operating system such asthe Microsoft Windows Operating System (OS), the IBM OS/2 operatingsystem, the MAC OS, or UNIX operating system. Those skilled in the artwill appreciate that the present invention may also be implemented onplatforms and operating systems other than those mentioned.

A preferred embodiment is written using Java, C, and the C++ languageand utilizes object-oriented programming methodology. Object-orientedprogramming (OOP) has become increasingly used to develop complexapplications. As OOP moves toward the mainstream of software design anddevelopment, various software solutions will need to be adapted to makeuse of the benefits of OOP. A need exists for these principles of OOP tobe applied to a messaging interface of an electronic messaging systemsuch that a set of OOP classes and objects for the messaging interfacecan be provided.

OOP is a process of developing computer software using objects,including the steps of analyzing the problem, designing the system, andconstructing the program. An object is a software package that containsboth data and a collection of related structures and procedures. Sinceit contains both data and a collection of structures and procedures, itcan be visualized as a self-sufficient component that does not requireother additional structures, procedures or data to perform its specifictask. OOP, therefore, views a computer program as a collection oflargely autonomous components, called objects, each of which isresponsible for a specific task. This concept of packaging data,structures, and procedures together in one component or module is calledencapsulation.

In general, OOP components are reusable software modules which presentan interface that conforms to an object model and which are accessed atrun-time through a component integration architecture. A componentintegration architecture is a set of architecture mechanisms which allowsoftware modules in different process spaces to utilize each otherscapabilities or functions. This is generally done by assuming a commoncomponent object model on which to build the architecture.

It is worthwhile to differentiate between an object and a class ofobjects at this point. An object is a single instance of the class ofobjects, which is often just called a class. A class of objects can beviewed as a blueprint, from which many objects can be formed.

OOP allows the programmer to create an object that is a part of anotherobject. For example, the object representing a piston engine is said tohave a composition-relationship with the object representing a piston.In reality, a piston engine comprises a piston, valves and many othercomponents; the fact that a piston is an element of a piston engine canbe logically and semantically represented in OOP by two objects.

OOP also allows creation of an object that "depends from" anotherobject. If there are two objects, one representing a piston engine andthe other representing a piston engine wherein the piston is made ofceramic, then the relationship between the two objects is not that ofcomposition. A ceramic piston engine does not make up a piston engine.Rather it is merely one kind of piston engine that has one morelimitation than the piston engine; its piston is made of ceramic. Inthis case, the object representing the ceramic piston engine is called aderived object, and it inherits all of the aspects of the objectrepresenting the piston engine and adds further limitation or detail toit. The object representing the ceramic piston engine "depends from" theobject representing the piston engine. The relationship between theseobjects is called inheritance.

When the object or class representing the ceramic piston engine inheritsall of the aspects of the objects representing the piston engine, itinherits the thermal characteristics of a standard piston defined in thepiston engine class. However, the ceramic piston engine object overridesthese ceramic specific thermal characteristics, which are typicallydifferent from those associated with a metal piston. It skips over theoriginal and uses new functions related to ceramic pistons. Differentkinds of piston engines will have different characteristics, but mayhave the same underlying functions associated with it (e.g., how manypistons in the engine, ignition sequences, lubrication, etc.). To accesseach of these functions in any piston engine object, a programmer wouldcall the same functions with the same names, but each type of pistonengine may have different/overriding implementations of functions behindthe same name. This ability to hide different implementations of afunction behind the same name is called polymorphism and it greatlysimplifies communication among objects.

With the concepts of composition-relationship, encapsulation,inheritance and polymorphism, an object can represent just aboutanything in the real world. In fact, our logical perception of thereality is the only limit on determining the kinds of things that canbecome objects in object-oriented software. Some typical categories areas follows:

Objects can represent physical objects, such as automobiles in atraffic-flow simulation, electrical components in a circuit-designprogram, countries in an economics model, or aircraft in anair-traffic-control system.

Objects can represent elements of the computer-user environment such aswindows, menus or graphics objects.

An object can represent an inventory, such as a personnel file or atable of the latitudes and longitudes of cities.

An object can represent user-defined data types such as time, angles,and complex numbers, or points on the plane.

With this enormous capability of an object to represent just about anylogically separable matters, OOP allows the software developer to designand implement a computer program that is a model of some aspects ofreality, whether that reality is a physical entity, a process, a system,or a composition of matter. Since the object can represent anything, thesoftware developer can create an object which can be used as a componentin a larger software project in the future.

If 90% of a new OOP software program consists of proven, existingcomponents made from preexisting reusable objects, then only theremaining 10% of the new software project has to be written and testedfrom scratch. Since 90% already came from an inventory of extensivelytested reusable objects, the potential domain from which an error couldoriginate is 10% of the program. As a result, OOP enables softwaredevelopers to build objects out of other, previously built, objects.

This process closely resembles complex machinery being built out ofassemblies and sub-assemblies. OOP technology, therefore, makes softwareengineering more like hardware engineering in that software is builtfrom existing components, which are available to the developer asobjects. All this adds up to an improved quality of the software as wellas an increased speed of its development.

Programming languages are beginning to fully support the OOP principles,such as encapsulation, inheritance, polymorphism, andcomposition-relationship. With the advent of the C++ language, manycommercial software developers have embraced OOP. C++ is an OOP languagethat offers a fast, machine-executable code. Furthermore, C++ issuitable for both commercial-application and systems-programmingprojects. For now, C++ appears to be the most popular choice among manyOOP programmers, but there is a host of other OOP languages, such asSmalltalk, common lisp object system (CLOS), and Eiffel. Additionally,OOP capabilities are being added to more traditional popular computerprogramming languages such as Pascal.

The benefits of object classes can be summarized, as follows:

Objects and their corresponding classes break down complex programmingproblems into many smaller, simpler problems.

Encapsulation enforces data abstraction through the organization of datainto small, independent objects that can communicate with each other.Encapsulation protects the data in an object from accidental damage, butallows other objects to interact with that data by calling the object'smember functions and structures.

Subclassing and inheritance make it possible to extend and modifyobjects through deriving new kinds of objects from the standard classesavailable in the system. Thus, new capabilities are created withouthaving to start from scratch.

Polymorphism and multiple inheritance make it possible for differentprogrammers to mix and match characteristics of many different classesand create specialized objects that can still work with related objectsin predictable ways.

Class hierarchies and containment hierarchies provide a flexiblemechanism for modeling real-world objects and the relationships amongthem.

Libraries of reusable classes are useful in many situations, but theyalso have some limitations. For example:

Complexity. In a complex system, the class hierarchies for relatedclasses can become extremely confusing, with many dozens or evenhundreds of classes.

Flow of control. A program written with the aid of class libraries isstill responsible for the flow of control (i.e., it must control theinteractions among all the objects created from a particular library).The programmer has to decide which functions to call at what times forwhich kinds of objects.

Duplication of effort. Although class libraries allow programmers to useand reuse many small pieces of code, each programmer puts those piecestogether in a different way. Two different programmers can use the sameset of class libraries to write two programs that do exactly the samething but whose internal structure (i.e., design) may be quitedifferent, depending on hundreds of small decisions each programmermakes along the way. Inevitably, similar pieces of code end up doingsimilar things in slightly different ways and do not work as welltogether as they should.

Class libraries are very flexible. As programs grow more complex, moreprogrammers are forced to reinvent basic solutions to basic problemsover and over again. A relatively new extension of the class libraryconcept is to have a framework of class libraries. This framework ismore complex and consists of significant collections of collaboratingclasses that capture both the small scale patterns and major mechanismsthat implement the common requirements and design in a specificapplication domain. They were first developed to free applicationprogrammers from the chores involved in displaying menus, windows,dialog boxes, and other standard user interface elements for personalcomputers.

Frameworks also represent a change in the way programmers think aboutthe interaction between the code they write and code written by others.In the early days of procedural programming, the programmer calledlibraries provided by the operating system to perform certain tasks, butbasically the program executed down the page from start to finish, andthe programmer was solely responsible for the flow of control. This wasappropriate for printing out paychecks, calculating a mathematicaltable, or solving other problems with a program that executed in justone way.

The development of graphical user interfaces began to turn thisprocedural programming arrangement inside out. These interfaces allowthe user, rather than program logic, to drive the program and decidewhen certain actions should be performed. Today, most personal computersoftware accomplishes this by means of an event loop which monitors themouse, keyboard, and other sources of external events and calls theappropriate parts of the programmer's code according to actions that theuser performs. The programmer no longer determines the order in whichevents occur. Instead, a program is divided into separate pieces thatare called at unpredictable times and in an unpredictable order. Byrelinquishing control in this way to users, the developer creates aprogram that is much easier to use. Nevertheless, individual pieces ofthe program written by the developer still call libraries provided bythe operating system to accomplish certain tasks, and the programmermust still determine the flow of control within each piece after it'scalled by the event loop. Application code still "sits on top of" thesystem.

Even event loop programs require programmers to write a lot of code thatshould not need to be written separately for every application. Theconcept of an application framework carries the event loop conceptfurther. Instead of dealing with all the nuts and bolts of constructingbasic menus, windows, and dialog boxes and then making these things allwork together, programmers using application frameworks start withworking application code and basic user interface elements in place.Subsequently, they build from there by replacing some of the genericcapabilities of the framework with the specific capabilities of theintended application.

Application frameworks reduce the total amount of code that a programmerhas to write from scratch. However, because the framework is really ageneric application that displays windows, supports copy and paste, andso on, the programmer can also relinquish control to a greater degreethan event loop programs permit. The framework code takes care of almostall event handling and flow of control, and the programmer's code iscalled only when the framework needs it (e.g., to create or manipulate aproprietary data structure).

A programmer writing a framework program not only relinquishes controlto the user (as is also true for event loop programs), but alsorelinquishes the detailed flow of control within the program to theframework. This approach allows the creation of more complex systemsthat work together in interesting ways, as opposed to isolated programs,having custom code, being created over and over again for similarproblems.

Thus, as is explained above, a framework basically is a collection ofcooperating classes that make up a reusable design solution for a givenproblem domain. It typically includes objects that provide defaultbehavior (e.g., for menus and windows), and programmers use it byinheriting some of that default behavior and overriding other behaviorso that the framework calls application code at the appropriate times.

There are three main differences between frameworks and class libraries:

Behavior versus protocol. Class libraries are essentially collections ofbehaviors that you can call when you want those individual behaviors inyour program. A framework, on the other hand, provides not only behaviorbut also the protocol or set of rules that govern the ways in whichbehaviors can be combined, including rules for what a programmer issupposed to provide versus what the framework provides.

Call versus override. With a class library, the code the programmerwrites instantiates objects and calls their member functions. It'spossible to instantiate and call objects in the same way with aframework (i.e., to treat the framework as a class library), but to takefull advantage of a framework's reusable design, a programmer typicallywrites code that overrides and is called by the framework. The frameworkmanages the flow of control among its objects. Writing a programinvolves dividing responsibilities among the various pieces of softwarethat are called by the framework rather than specifying how thedifferent pieces should work together.

Implementation versus design. With class libraries, programmers reuseonly implementations, whereas with frameworks, they reuse design. Aframework embodies the way a family of related programs or pieces ofsoftware work. It represents a generic design solution that can beadapted to a variety of specific problems in a given domain. Forexample, a single framework can embody the way a user interface works,even though two different user interfaces created with the sameframework might solve quite different interface problems.

Thus, through the development of frameworks for solutions to variousproblems and programming tasks, significant reductions in the design anddevelopment effort for software can be achieved. A preferred embodimentof the invention utilizes Hypertext Markup Language (HTML) to implementdocuments on the Internet together with a general-purpose securecommunication protocol for a transport medium between the client and themerchant. HTML is a simple data format used to create hypertextdocuments that are portable from one platform to another. HTML documentsare Standard Generalized Markup Language (SGML) documents with genericsemantics that are appropriate for representing information from a widerange of domains. HTML has been in use by the World-Wide Web globalinformation initiative since 1990. HTML is an application of ISOStandard 8879:1986 Information Processing Text and Office Systems;Standard Generalized Markup Language (SGML).

To date, Web development tools have been limited in their ability tocreate dynamic Web applications which span from client to server andinteroperate with existing computing resources. Until recently, HTML hasbeen the dominant technology used in development of Web-based solutions.However, HTML has proven to be inadequate in the following areas:

Poor performance;

Restricted user interface capabilities;

Can only produce static Web pages;

Lack of interoperability with existing applications and data; and

Inability to scale.

Sun Microsystem's Java language solves many of the client-side problemsby:

Improving performance on the client side;

Enabling the creation of dynamic, real-time Web applications; and

Providing the ability to create a wide variety of user interfacecomponents.

With Java, developers can create robust User Interface (UI) components.Custom "widgets" (e.g. real-time stock tickers, animated icons, etc.)can be created, and client-side performance is improved. Unlike HTML,Java supports the notion of client-side validation, offloadingappropriate processing onto the client for improved performance.Dynamic, real-time Web pages can be created. Using the above-mentionedcustom UI components, dynamic Web pages can also be created.

Sun's Java language has emerged as an industry-recognized language for"programming the Internet." Sun defines Java as: "a simple,object-oriented, distributed, interpreted, robust, secure,architecture-neutral, portable, high-performance, multithreaded,dynamic, buzzword-compliant, general-purpose programming language. Javasupports programming for the Internet in the form ofplatform-independent Java applets." Java applets are small, specializedapplications that comply with Sun's Java Application ProgrammingInterface (API) allowing developers to add "interactive content" to Webdocuments (e.g. simple animations, page adornments, basic games, etc.).Applets execute within a Java-compatible browser (e.g. NetscapeNavigator) by copying code from the server to client. From a languagestandpoint, Java's core feature set is based on C++. Sun's Javaliterature states that Java is basically "C++, with extensions fromObjective C for more dynamic method resolution".

Another technology that provides similar function to Java is provided byMicrosoft and ActiveX Technologies, to give developers and Web designerswherewithal to build dynamic content for the Internet and personalcomputers. ActiveX includes tools for developing animation, 3-D virtualreality, video and other multimedia content. The tools use Internetstandards, work on multiple platforms, and are being supported by over100 companies. The group's building blocks are called ActiveX Controls,small, fast components that enable developers to embed parts of softwarein hypertext markup language (HTML) pages. ActiveX Controls work with avariety of programming languages including Microsoft Visual C++, BorlandDelphi, Microsoft Visual Basic programming system and, in the future,Microsoft's development tool for Java, code named "Jakarta." ActiveXTechnologies also includes ActiveX Server Framework, allowing developersto create server applications. One of ordinary skill in the art willreadily recognize that ActiveX could be substituted for Java withoutundue experimentation to practice the invention.

FIG. 2 depicts a client computer 210 operating a network client program,such as a web browser, in communication with server computer 215. Server215 may be, for example, a hypertext transport protocol ("HTTP") server.Server 215 is named "SYSA". This denotes that the server is addressedwith a name such as SYSA, or is located externally to the client'snetwork, for example, in a system with the name SYSA.COM. Server SYSA215 is in communication with client computer 210 using a communicationslink 220 operating, for example, using the HTTP protocol and theTransmission Control Protocol and Internet Protocol ("TCP" and "IP,"respectively, or collectively "TCP/IP"). HTTP is described in R.Fielding, et al., Hypertext Transfer Protocol: HTTP/1.1 (Jun. 3, 1996)(draft), the disclosure of which is hereby incorporated by reference.TCP is described in Information Sciences Institute, RFC 793:Transmission Control Protocol DARPA Internet Program ProtocolSpecification (September 1981), the disclosure of which is herebyincorporated by reference. IP is described in Information SciencesInstitute, RFC 791: Internet Protocol DARPA Internet Program ProtocolSpecification (September 1981), the disclosure of which is herebyincorporated by reference.

As depicted in FIG. 2, server SYSA 215 is transferring a copy ofdocument 225, named PAGE1.HTML, to client computer 210, in response to aprevious HTTP GET request (not shown) by client computer 210. Clientcomputer 210 is equipped with a cache file 230 indexed by a cache table235. Cache table 235 is a table used to index cache 230. Cache table 235comprises a plurality of rows 238 and columns 240. Each row 238 is usedto describe a cached element, such as downloaded HTML document 225.Columns 240 include a URI column 245, an OID column 247, and a cachepointer column 249. The use of the cache table 235 will be described infurther detail below, however, in summary, URI column 245 contains arepresentation of a location code of a cached resource. Typically thiswill be a representation of a Uniform Resource Identifier ("URI"), suchas a Uniform Resource Locator ("URL"), or any other indicator of thelocation of the cached resource. The syntax and semantics of URLs aredescribed in Berners-Lee, et al., RFC 1738: Uniform Resource Locators(URL) (December 1994), the disclosure of which is hereby incorporated byreference.

OID column 247 contains an object identifier, if any for the cachedresource, or is null if no object identifier is associated with thecached resource. Cache pointer column 249 contains a pointer to thecache 230 element in the cache. This may be, for example, the name of afile located within a specific predetermined cache directory or cachedevice, or it may be a specification of a location within a larger fileof a particular portion of the file corresponding to the cachedresource. In addition, the cache table 235 will typically containadditional fields used to manage and optimize a cache table, such asindicators of whether a particular cache row 238 is in use, itsfrequency of reference, etc. Such fields are not a portion of thepresent invention and are not herein described.

As depicted in FIG. 2, client computer 210 transfers a copy of document225 to cache 230 as shown by arrow 260. As depicted in FIG. 2, a copy ofdocument 225 is cached at location 265, and cache table row 238-1 isupdated to reflect the cache. In particular, for cache table row 238-1,URI column 245 is set to the value of the URI for the retrieveddocument. Column 247 is set to null because, as will be described infurther detail, no OID has been associated with this resource. Column249 has been set to point to cached element 265.

FIG. 3 depicts an example of downloaded document 225. In the depictedexample, the file is in Hypertext Markup Language ("HTML"). The HTMLformat is described in Berners-Lee, et al., RFC 1866: Hypertext MarkupLanguage--2.0 (November 1995), which is hereby incorporated byreference. The HTML document 225 comprises a plurality of HTML tags 310and text 320 to be rendered and displayed. With the exception of theapplet tag 330, all of HTML tags 310 are known and described inBerners-Lee, and are not further described herein. Applet tag 330 is anHTML tag that is used to specify an applet that should be loaded andexecuted when HTML document 225 is rendered and displayed. Specificallyapplet tag 330 is an extension to HTML devised by Sun Microsystems, Inc.of Mountain View, Calif. Typically, applets are written in aplatform-independent object-oriented language such as Sun Microsystem'sJava language. A Java applet is generally transported in the Sun'sCLASS-format file. Applet tag 330 contains four parameters: CODEparameter 340, WIDTH parameter 342, HEIGHT parameter 344, and OIDparameter 350. The CODE, WIDTH, and HEIGHT parameters are known in theart. The CODE parameter 340 is used to specify the name of a programfile to be downloaded from the same server location as HTML document225. In the example depicted, CODE parameter 340 indicates that theprogram named FOO.CLASS is to be obtained from the SYSA server 215because the web page was part of the HTML information. WIDTH parameter342 and HEIGHT parameter 344 indicate the size of screen area where theoutput of the applet will be displayed. In the example shown, an area300 pixels wide and 100 pixels high is specified.

The OID parameter 350 is not previously known. OID parameter 350 is usedto specify an object identifier. The object identifier is a uniqueidentifier associated with a particular object. The OID parameter 350associated with a particular object is guaranteed to be unique. Apreferred method of specifying the OID parameter 350 is using AbstractSyntax Notation One ("ASN.1"), as described in ISO 8824, and the ASN.1Basic Encoding Rules ("BER") as described in ISO 8825. ASN.1 and BERprovide for an "object identifier" datatype.

The ASN.1 object identifier is in the form of a series of integersseparated by decimal points. Each integer represents a node on an ASN.1object identifier tree. The ASN.1 object identifier tree is a structurewith a root node, arcs beneath that node to other nodes, with arcsbeneath them and so on. As specified in the ASN.1 standard, each node isassigned to some responsible body that allocates arcs and nodes beneathit. The body ensures that all the arcs beneath its node are numberedsequentially starting from 0 or 1, and that each node beneath it iseither assigned to some responsible body or is assigned to name aparticular object. In the example shown, the OID parameter 350 has avalue of 1.1.999999.72.6.3. The first integer, 1, has been allocated tothe International Standards Organization ("ISO"). The ISO, therefore,has the responsibility for allocating every OID beginning with theinteger 1. The second integer 1 indicates that this object is within anASN.1 hierarchy allocated to a "registration authority" ("RA"). Aregistration authority is an organization that is responsible for theallocation of any further ASN.1 OIDs within its name space. In theexample shown, the fictitious registration authority code 999999 is usedto depict that a particular responsibility authority having beenassigned a code 999999 is responsible for any object identifiers belowthis level. The remaining integers (72, 6, and 3) are arbitrary integersorganized according to a functional scheme selected by the registrationauthority corresponding to RA code 999999, and guaranteed by theregistration authority to be unique in the world. ASN.1, BER, the ASN.1object identifier, and the ASN.1 object identifier tree are knownconstructs and are described in their respective standards and, forexample, in Larmouth, Understanding OSI(International Thomson ComputerPress, 1996), pp. 151-160, hereby incorporated by reference.

It will be seen from FIG. 3, therefore, that the desired applet may bereferred to either by a particular known location (CODE parameter 340,file name FOO.CLASS on server SYSA 215) or by its ASN.1 objectidentifier (1.1.999999.72.6.3).

FIG. 4 depicts an operation of obtaining the applet specified by applettag 330 in document 225 of FIG. 3. In the example depicted, the appletspecified by applet tag 330 has not been previously obtained, andtherefore is unavailable from cache 230 of FIG. 2. Therefore, asdepicted in FIG. 4, client computer 210 issues an HTTP GET request forthe resource named FOO.CLASS, as specified by the CODE parameter 340 ofdocument 225 (FIG. 3) on connection 220 to server SYSA 215.

FIG. 5 depicts the server response to the GET request illustrated inFIG. 4. Server SYSA 215 transmits a copy 510 of the FOO.CLASS object oncommunications line 220 to client computer 210. Client computer 210places a copy of received file 510 in cache 230 as cache element 520.Client computer 210 then updates cache table 235 to reflect the cacheupdate. Specifically, client computer 210 updates row 238-2 to reflectnewly added element 520.

Client computer 210 updates cache table row 238-2 as follows. URI column245 is set to the value of the URI from which the file was retrieved.OID column 247 is updated with the object identifier value for theobject received, which in this case is the OID parameter 350 of FIG. 3.Finally, column 249 is updated with a pointer to the cache element 520within cache 230.

Generally, an object that is received from a server may have an OIDencoded into it, so that a client computer such as client computer 210can discover the object's OID even if no OID had been specified prior tothe transfer. The OID may be encoded, for example, in a predeterminedposition or field within the object, or may be stored on the serverseparately from the object and included by the server by encoding itinto the HTTP GET response.

The Java CLASS format is highly structured and is particularlywell-suited for the purpose of encoding an OID into an applet file. TheJava CLASS format is described in Sun Microsystem's Java Virtual MachineSpecification, Release 1.0 Beta DRAFT (Aug. 21, 1995), the disclosure ofwhich is hereby incorporated by reference.

Each class file contains the compiled version of either a Java class ora Java interface. An interpreter or "virtual machine" designed toexecute a Java applet supports all class files that conform to thisformat.

A Java class file comprises a stream of 8-bit bytes. All 16-bit and32-bit quantities are constructed by reading in two or four 8-bit bytes,respectively. The bytes are joined together in network (big-endian)order, where the high bytes come first. This format is supported by theJava java.io.DataInput and java.io.DataOutputinterfaces, and classessuch as java.io.DataInputStream andjava.io.DataOutputStream andjava.io.DataOutputStream.

The class file format is described here using a structure notation.Successive fields in the structure appear in the external representationwithout padding or alignment. Variable size arrays, often of variablesized elements are called tables and are commonplace in thesestructures. The types u1, u2, and u4 mean an unsigned one-, two-, orfour-byte quantity, respectively, which are read by method such asreadUnsignedByte, readUnsignedShort and readInt of thejava.io.DataInputinterface.

FIG. 5A depicts the format of a class file 560, which is structured asfollows:

    ______________________________________                                        ClassFile{                                                                    u4 magic;                                                                     u2 minor.sub.-- version;                                                      u2 major.sub.-- version;                                                      u2 constant.sub.-- pool.sub.-- count;                                         cp.sub.-- info constant.sub.-- pool[constant.sub.-- pool.sub.-- count-        1];                                                                           u2 access.sub.-- flags;                                                       u2 this.sub.-- class;                                                         u2 super.sub.-- class;                                                        u2 interfaces.sub.-- count;                                                   u2 interfaces[interfaces.sub.-- count];                                       u2 fields.sub.-- count;                                                       field.sub.-- info fields[fields.sub.-- count];                                u2 methods.sub.-- count;                                                      method.sub.-- info methods[methods.sub.-- count];                             u2 attributes.sub.-- count;                                                   attribute.sub.-- info attributes[attribute.sub.-- count];                     ______________________________________                                    

magic

The "magic" field 561 is four bytes in length and is used to identifythe file as a Java class-format file. The magic field has the value0×CAFEBABE.

major₋₋ version and major version

The minor₋₋ version field 562 and major₋₋ version field 563 contain theversion number of the Java compiler that produced this class file. Thecombination of the two fields may be interrogated by a virtual machineto determine whether it is capable of executing the applet. Animplementation of the virtual machine will normally support some rangeof minor version numbers 0-n of a particular major version number. Ifthe minor version number is incremented, the new code won't run on theold virtual machines, but it is possible to make a new virtual machinewhich can run versions up to version number n+1. A change of the majorversion number indicates a major incompatible change, one that requiresa different virtual machine that may not support the old major versionin any way.

constant₋₋ pool₋₋ count

The constant₋₋ pool₋₋ count field 564 indicates the number of entries inthe constant₋₋ pool 565 in the class file.

constant₋₋ pool

The constant₋₋ pool 565 is a table of values. The values in theconstant₋₋ pool 565 comprise various string constants, classnames, fieldnames, and others that are referred to by the class structure or by theexecutable code in the applet. The first constant pool entry, denoted asconstant₋₋ pool[0], is always unused by the compiler, and may be used byan implementation for any purpose.

Each of the constant₋₋ pool entries 1 through constant₋₋ pool₋₋ count564 minus one is a variable-length entry, whose format is indicated bythe first "tag" byte, according to the following table:

    ______________________________________                                        Value Constant Type      Meaning                                              ______________________________________                                        1     CONSTANT.sub.-- Utf8                                                                             utf-8 format string                                  2     CONSTANT.sub.-- Unicode                                                                          unicode format string                                3     CONSTANT.sub.-- Integer                                                                          integer                                              4     CONSTANT.sub.-- Float                                                                            floating point                                       5     CONSTANT.sub.-- Long                                                                             long integer                                         6     CONSTANT.sub.-- Double                                                                           double floating point                                7     CONSTANT.sub.-- Class                                                                            class                                                8     CONSTANT.sub.-- String                                                                           string                                               9     CONSTANT.sub.-- Fieldref                                                                         field reference                                      10    CONSTANT.sub.-- Methodref                                                                        method reference                                     11    CONSTANT.sub.-- InterfaceMethodref                                                               interface method reference                           12    CONSTANT.sub.-- NameAndType                                                                      name and type                                        ______________________________________                                    

A utf-8 format string constant₋₋ pool entry represents a constantcharacter string value. Utf-8 strings are encoded so that stringscontaining only non-null ASCII characters, can be represented using onlyone byte per character, but characters of up to 16 bits can still berepresented.

All characters in the range 0×0001 to 0×007F are represented by a singlebyte, in which bit 0 is set to binary `0` and in which bits 1-7represent the ASCII code 0×0001 to 0×007F, respectively. The nullcharacter 0×0000 and characters in the range 0×0080 to 0×07FF arerepresented by a pair of two bytes, or 16 bits, denoted here as bits0-15. Bits 0-2 are set to binary `110` and bits 8-9 are set to binary`10`. The remaining eleven bits 3-7 and 10-15 correspond respectively tothe low-order eleven bits in the character to be encoded.

Characters in the range 0×0800 to 0×FFFF are represented by three bytes,or 24 bits, denoted here as bits 0-23. Bits 0-3, 8-9 and 16-17 are setto binary values `1110`, `10`, and `10`, respectively. The remaining 16bits 4-7, 10-15 and 18-23 correspond to the 16 bits in the character tobe encoded.

The null character 0×00 is encoded in two-byte format rather thanone-byte, with the result that encoded strings never have embeddednulls. Only one-byte, two-byte, and three-byte formats are used; longerutf-8 formats are unrecognized.

A utf-8 string is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Utf8.sub.-- info{                                             u1 tag;                                                                       u2 length;                                                                    u1 bytes[length];                                                             ______________________________________                                    

The tag field has the constant value 0×`0001` indicating a utf-8 encodedstring. The length field is a two-byte field indicating the length ofthe string. The bytes field is the encoded string.

A UNICODE string constant₋₋ pool entry represents a constant unencodedcharacter string value. A UNICODE string is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Unicode.sub.-- info{                                          u1 tag;                                                                       u2 length;                                                                    u1 bytes[length];                                                             ______________________________________                                    

The tag field has the constant value 0×`0002` indicating aunicode-format string. The length field is a two-byte field indicatingthe length of the string. The bytes field is the string value.

An integer constant₋₋ pool entry represents a four-byte integer. Theconstant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Integer.sub.-- info{                                          u1 tag;                                                                       u4 bytes;                                                                     ______________________________________                                    

The tag field has the constant value 0×`0003` indicating a integer. Thebytes field is the integer value.

A float constant₋₋ pool entry represents a four-byte floating-pointnumber. The constant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Float.sub.-- info{                                            u1 tag;                                                                       u4 bytes;                                                                     ______________________________________                                    

The tag field has the constant value 0×`0004` indicating afloating-point number. The bytes field is the floating-point value.

A long integer constant₋₋ pool entry represents an eight-byte integer.The constant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Long.sub.-- info{                                             u1 tag;                                                                       u4 high.sub.-- bytes;                                                         u4 low.sub.-- bytes;                                                          ______________________________________                                    

The tag field has the constant value 0×`0005` indicating a long integer.The high₋₋ bytes and low₋₋ bytes fields together make up the integervalue. A long integer constant₋₋ pool entry takes up two spots in theconstant₋₋ pool 565. If this is the nth entry in the constant₋₋ pool565, then the next entry will be numbered n+2.

A double float constant₋₋ pool entry represents an eight-bytefloating-point number. The constant₋₋ pool entry is structured asfollows:

    ______________________________________                                        CONSTANT.sub.-- Double.sub.-- info{                                           u1 tag;                                                                       u4 high.sub.-- bytes;                                                         u4 low.sub.-- bytes;                                                          ______________________________________                                    

The tag field has the constant value 0×`0006` indicating a doublefloating-point number. The high₋₋ bytes and low₋₋ bytes fields togethermake up the floating-point value. A double float constant₋₋ pool entrytakes up two spots in the constant pool 565. If this is the nth entry inthe constant pool 565, then the next entry will be numbered n+2.

A class constant₋₋ pool entry represents a Java class or an interface.The constant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Class.sub.-- info{                                            u1 tag;                                                                       u2 name.sub.-- index;                                                         ______________________________________                                    

The tag field has the constant value 0×`0007` indicating a class. Thename₋₋ index field is a subscript into the constant₋₋ pool 565, to autf-8 format string constant that gives the string name of the class.

A string constant₋₋ pool entry represents Java objects of the built-inJava type "String." The constant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- String.sub.-- info{                                           u1 tag;                                                                       u2 string.sub.-- index;                                                       ______________________________________                                    

The tag field has the constant value 0×`0008` indicating a string. Thestring₋₋ index field is a subscript into the constant₋₋ pool 565, to autf-8 format string constant that gives the value to which theString-type object is initialized.

A field constant₋₋ pool entry, method reference constant₋₋ pool entry,and interface method reference constant₋₋ pool entry representreferences to Java fields, methods, and interface methods, respectively.The constant₋₋ pool entries are structured as follows:

    ______________________________________                                        CONSTANT.sub.-- Fieldref.sub.-- info{                                         u1 tag;                                                                       u2 class.sub.-- index;                                                        u2 name.sub.-- and.sub.-- type.sub.-- index;                                  CONSTANT.sub.-- Methodref.sub.-- info{                                        u1 tag;                                                                       u2 class.sub.-- index;                                                        u2 name.sub.-- and.sub.-- type.sub.-- index;                                  }                                                                             CONSTANT.sub.-- InterfaceMethodref.sub.-- info{                               u1 tag;                                                                       u2 class.sub.-- index;                                                        u2 name.sub.-- and.sub.-- type.sub.-- index;                                  }                                                                             ______________________________________                                    

The tag field has the constant value 0×`0009`, 0×`000A`, or 0×`000B`,indicating a field reference, method reference, or interface methodreference, respectively. The class₋₋ index field is a subscript into theconstant₋₋ pool 565, to a class constant that is used to identify thename of the class or interface containing the field or method. Thename₋₋ and₋₋ type₋₋ indexfield is a subscript into the constant₋₋ pool565, to a NameAndType constant that is used to identify the name andsignature of the field or method.

A NameAndType constant₋₋ pool entry represents a field or method withoutindicating the class to which the name or field, as the case may be,belongs. The constant₋₋ pool entry is structured as follows:

    ______________________________________                                        CONSTANT.sub.-- NameAndType.sub.-- info{                                      u1 tag;                                                                       u2 name.sub.-- index;                                                         u2 signature.sub.-- index;                                                    ______________________________________                                    

The tag field has the constant value 0×`000C` indicating a NameAndTypeentry. The name₋₋ index field is a subscript into the constant₋₋ pool565, to a utf-8 format string constant that gives the name of the fieldor method. The signature₋₋ index field is a subscript into theconstant₋₋ pool 565, to a utf-8 format string constant that gives asignature of the field or method. The signature, in this context, refersto a string that represents a type of a method, field or array. Thefield signature represents the value of an argument to a function or thevalue of a variable. A return-type signature represents the return valuefrom a method. An argument signature represents an argument passed to amethod. A method signature comprises one or more arguments signaturesand a return signature, thereby representing the arguments expected by amethod, and the value that it returns.

The structure and self-referential nature of the cell pool therebyprovides great flexibility in implementation of data encoded in anapplet file.

access₋₋ flags

The access₋₋ flags field 566 contains a mask of up to sixteen modifiersused with class, method, and field declarations. The same encoding isused on similar fields in field₋₋ info and method₋₋ info as describedbelow. The access₋₋ flags field is encoded as follows:

    ______________________________________                                        Flag Name     Value   Meaning     Used By                                     ______________________________________                                        ACC.sub.-- PUBLIC                                                                           0x0001  Visible to  Class, Method,                                                    everyone    Variable                                    ACC.sub.-- PRIVATE                                                                          0x0002  Visible only to                                                                           Method,                                                           the defining class                                                                        Variable                                    ACC.sub.-- PROTECTED                                                                        0x0004  Visible to  Method,                                                           subclasses  Variable                                    ACC.sub.-- STATIC                                                                           0x0008  Variable or Method,                                                           method is static                                                                          Variable                                    ACC.sub.-- FINAL                                                                            0x0010  No further  Class, Method,                                                    subclassing,                                                                              Variable                                                          overriding, or                                                                assignment after                                                              initialization                                          ACC.sub.-- SYNCHRONIZED                                                                     0x0020  Wrap use in Method                                                            monitor lock                                            ACC.sub.-- VOLATILE                                                                         0x0040  Can't cache Variable                                    ACC.sub.-- TRANSIENT                                                                        0x0080  Not to be written                                                                         Variable                                                          or read by a                                                                  persistent object                                                             manager                                                 ACC.sub.-- NATIVE                                                                           0x0100  Implemented in a                                                                          Method                                                            language other                                                                than Java                                               ACC.sub.-- INTERFACE                                                                        0x0200  Is an interface                                                                           Class                                       ACC.sub.-- ABSTRACT                                                                         0x0400  No body provided                                                                          Class, Method                               ______________________________________                                    

this₋₋ class

The this₋₋ class field 567 is an index into the constant pool 565;constant₋₋ pool[this₋₋ class]must be of type CONSTANT₋₋ class.

super₋₋ class

The super₋₋ class 568 field is an index into the constant₋₋ pool 565. Ifthe value of super₋₋ class field 568 is nonzero, then constant₋₋pool[super₋₋ class]must be a class, and gives the index of this class'ssuperclass (that is, the class from which the present class is derived)in the constant₋₋ pool 565. If the value of super₋₋ class field 568 iszero, then the class being defined must be java.lang.Object, and it hasno superclass.

interfaces₋₋ count

The interfaces₋₋ count field 569 gives the number of interfaces thatthis class implements.

interfaces table

Each value in interfaces table 570 is an index into the constant₋₋ pool565. If an table value is nonzero(interfaces[i]!=0, where0<=i<interfaces₋₋ count), then constant₋₋ pool[interfaces[i]]must be aninterface that this class implements.

fields₋₋ count

The fields₋₋ count field 571 gives the number of instance variables,both static and dynamic, defined by the thisclass field. The fieldstable 572 includes only those variables that are defined explicitly bythis class. It does not include those instance variables that areaccessible from this class but are inherited from superclasses.

fields table

Each value in the fields table 572 is a more complete description of afield in the class. Each field is described by a variable length field₋₋info structure. The format of this structure is as follows:

    ______________________________________                                        field.sub.-- info{                                                            u2 access.sub.-- flags;                                                       u2 name.sub.-- index;                                                         u2 signature.sub.-- index;                                                    u2 attributes.sub.-- count;                                                   attribute.sub.-- info attributes[attribute.sub.-- count];                     ______________________________________                                    

The access₋₋ flags field is a set of sixteen flags used by classes,methods, and fields to describe various properties and how they many beaccessed by methods in other classes. This field has the same names,values and meanings as the access₋₋ flags field 566 previouslydisclosed.

The possible flags that can be set for a field are ACC₋₋ PUBLIC, ACC₋₋PRIVATE, ACC₋₋ PROTECTED, ACC₋₋ STATIC, ACC₋₋ FINAL, ACC₋₋ VOLATILE, andACC₋₋ TRANSIENT. At most one of ACC₋₋ PUBLIC, ACC₋₋ PROTECTED, and ACC₋₋PRIVATE can be set for any method.

The name₋₋ index field is a subscript used to index into the constant₋₋pool 565 indicating a CONSTANT₋₋ Utf8 string, which is the name of thefield.

The signature₋₋ index field is a subscript that is used to index intothe constant₋₋ pool 565 to indicate a CONSTANT₋₋ Utf8 string, which isthe signature of the field.

The attributes₋₋ count field indicates the number of additionalattributes about this field.

The attributes field represents the attributes of a particular fieldrepresented by the field₋₋ info structure. A field can have any numberof optional attributes associated with it. For example, the"ConstantValue" attribute, which indicates that this field is a staticnumeric constant, indicates the constant value of that field.

methods₋₋ count

The methods₋₋ count field 573 indicates the number of methods, bothstatic and dynamic, defined by this class. This table only includesthose methods that are explicitly defined by this class. It does notinclude inherited methods.

methods table

Each value in the methods table 574 is a more complete description of amethod in the class. Each method is described by a variable lengthmethod₋₋ info structure. The format of this structure is as follows:

    ______________________________________                                        method.sub.-- info{                                                           u2 access.sub.-- flags;                                                       u2 name.sub.-- index;                                                         u2 signature.sub.-- index;                                                    u2 attributes.sub.-- count;                                                   attribute.sub.-- info attributes[attribute.sub.-- count];                     ______________________________________                                    

The access₋₋ flags field is a set of sixteen flags used by classes,methods, and fields to describe various properties and how they many beaccessed by methods in other classes. This field has the same names,values and meanings as the access₋₋ flags field 566 previouslydisclosed. The possible fields that can be set for a method are ACC₋₋PUBLIC, ACC₋₋ PRIVATE, ACC₋₋ PROTECTED, ACC₋₋ STATIC, ACC₋₋ FINAL, ACC₋₋SYNCHRONIZED, ACC₋₋ NATIVE, and ACC₋₋ ABSTRACT. At most one of ACC₋₋PUBLIC, ACC₋₋ PROTECTED, and ACC₋₋ PRIVATE can be set for any method.

The name₋₋ index field is a subscript used to index into the constant₋₋pool 565 indicating a CONSTANT₋₋ Utf8 string, which is the name of themethod.

The signature₋₋ index field is a subscript that is used to index intothe constant₋₋ pool 565 to indicate a CONSTANT₋₋ Utf8 string, which isthe signature of the method.

The attributes₋₋ count field indicates the number of additionalattributes about this method.

The attributes field represents the attributes of a particular methodrepresented by the method₋₋ info structure. A method can have any numberof optional attributes associated with it. Each attribute has a name,and other additional information. For example, the "Code" attributedescribes the bytecodes that are executed to perform this method, andthe "Exceptions" attribute describes the Java Exceptions that aredeclared to result from the execution of the method.

attributes₋₋ count

The attributes₋₋ count field 575 indicates the number of additionalattributes about this class.

attributes table

The attributes table 576 defines the attributes associated with theclass. A class can have any number of optional attributes associatedwith it. For example, the "SourceFile" attribute indicates the name ofthe source file from which this class file was compiled.

Because of the highly structured nature of the Java CLASS file format,the file format is particularly well-suited for the purpose of encodingan OID into an applet file. In particular, the OID may be encoded as aattribute associated with the applet class. The structured nature of theCLASS format facilitates retrieval of the OID attribute. The OID may bemost easily implemented as an attribute associated with the class andencoded into attribute table 576. It will be recalled that attributetable 576 includes all attributes associated with this class, such asthe "SourceFile" attribute. An additional attribute the OID attributemay be implemented to provide the OID for the applet.

The OID attribute has the following format:

    ______________________________________                                        OID.sub.-- attribute{                                                         u2 attribute.sub.-- name.sub.-- index;                                        u4 attribute.sub.-- length;                                                   u2 oid.sub.-- index;                                                          ______________________________________                                    

The attribute₋₋ name₋₋ indexfield is a two-byte field that provides anindex into constant₋₋ pool 565. The value in the attribute₋₋ name₋₋indexfield is used to select a constant₋₋ pool entry of type CONSTANT₋₋Utf8 string encoding the character string "OID".

The attribute₋₋ length field is a four-byte field containing the value0×0002, indicating that the following oid₋₋ index field is two bytes inlength.

The oid₋₋ index field is a two-byte field that that provides an indexinto constant₋₋ pool 565. The value in the oid₋₋ index field is used toselect a constant₋₋ pool entry of type CONSTANT₋₋ Utf8 string. Thestring encoded in the selected CONSTANT₋₋ Utf8 string is the ASN.1 OIDvalue associated with this applet.

Another file type that lends itself to including an embedded OID is theCompuServe Information Service's Graphics Interchange Format (GIF). TheGIF file format is described in CompuServe, Inc., Graphics InterchangeFormat Version 89a (modified)(Jan. 9, 1995), the disclosure of which ishereby incorporated by reference. The GIF specification is primarilygeared to the encoding of compressed data that may be used to representa graphic image. The GIF format specifies the optional inclusion of anApplication Extension, a data area that may include application-specificdata. FIG. 5B depicts the format of an Application Extension data area580.

One-byte Extension Introducer 581 has a value 0×21, which defines thedata area as an extension. One-byte Application Extension Label 582 hasa value 0×FF, which identifies the extension as an ApplicationExtension. Blocksize 583 is a one-byte field that defines the size ofthe block, up to but not including Application Data 586, and has thevalue 0×0B (decimal 11). Application Identifier 584 is an eight-byteprintable ASCII sequence used to identify the application associatedwith the Application Extension. A value of `ASN.1OID` indicates that theApplication Extension 580 is used to encode an ASN.1OID. ApplicationAuthorization Code 585 is three bytes in length, and is optionally usedby an application to validate the Application Identifier 585. It neednot be employed in the present invention. Application data 586 isapplication-dependent data, and may be used to encode an OID. BlockTerminator 587 is a one-byte field containing a value 0×00, and is usedto indicate the end of the Application Extension 580.

Application data 586 may be used to encode an OID in the following form:

    ______________________________________                                        ASN.1.sub.-- OID{                                                             int length;                                                                   string oid;                                                                   ______________________________________                                    

Where the length field is an integer that indicates the length of theOID, and the oid field is a character value of the OID associated withthe GIF file.

In the preferred embodiment, following the extraction of the OID fromthe received file, client computer 210 must perform some verification ofreceived object 510, to validate that the received object 510 does, infact, correspond to the OID specified and allocated by the softwaredistributor that has responsibility for the OID. Typically, this may beperformed by transmitting with the object 510 a digital signatureassociated with the file. Numerous methods of calculating digitalsignatures are known. According to one such methodology, a "messagedigest" is first calculated based upon the contents of the file to bedigitally signed, in this case file 510. A message digest is thefixed-length result when a variable length message or file is providedto a one-way hashing function. A message digest helps verify that amessage has not been altered, because a message digest calculated on analtered message would not equal the message digest on the unalteredmessage. After the message digest is calculated, the digest is encryptedusing a private key associated with the software distributor. Arecipient of the message (e.g., file 510) and the encrypted messagedigest for that message may validate the content of the message by firstcalculating its own version of the message digest, and then decryptingthe received message digest using a public key maintained and widelydistributed by the software distributor, and known to be valid andassociated with that software distributor because it can be validatedwith a known registration authority. If the calculated message digest isequal to the decrypted received message digest, the message is known tobe unaltered.

Message validation may be performed using any of a number of widelyavailable public key cryptography methods. Examples of such methodsinclude Private Communications Technology ("PCT") from Microsoft, Inc.,Secure Hyper-Text Transport Protocol ("SHTTP") from Theresa Systems,Shen, Kerberos, Photuris, Pretty Good Privacy ("PGP") and Ipv6.

It should be noted that validation is a required step only in anuntrusted environment, where the status of received objects is not knownor is not trusted. In a trusted environment, in which the integrity ofthe object IDs and their associated resources are known to be valid, orwhere there is a high degree of confidence in the OID-resource coherencyand the consequences of incoherence are minimal, the validation step maybe ignored. For example, in a non-mission-critical environment in whichall clients and all servers are under the control of the authorityresponsible for all of the OIDs, the validation of received objectsmight be ignored.

FIG. 6 depicts transmission of a second HTML document 610 from a secondHTTP server 615, named SYSB, to client computer 210 over communicationslink 620. Upon receipt of document 610, client computer 210 updatescache 230 as previously described. That is, client computer 210 adds anew row 238-3 in which URI column 245 is set to the URI for file 610,OID column 247 is set to null, and cache pointer column 249 is set topoint to cache element 620.

FIG. 7 depicts an example of HTML document 610 illustrative of thepresent invention. As with previous document 225, HTML document 610comprises a number of HTML tags 710, including applet tag 730. Applettag 730 comprises a CODE parameter 740, a WIDTH parameter 742, and aHEIGHT parameter 744, and OID parameter 750. It will be noted that thevalues for the CODE, WIDTH, and HEIGHT parameters in applet tag 730 arearbitrary and are not the same as those previously shown for Applet tag330 in FIG. 3. However, it will be noted that the OID parameter 750 hasa value of 1.1.999999.72.6.3, which is identical to the previouslyobtained object, as can be seen by reference to cache table row 238-2 inFIG. 5. Therefore, it will be seen as shown in FIG. 6, that there aretwo different methods by which client computer 210 may obtain a copy ofthe desired object. It may issue a second HTTP request to server SYSB615 using the URI, or it may retrieve the corresponding cache element620 from the cache 230.

FIG. 8 depicts the process by which client computer 210 obtains a copyof the desired object. Using the OID parameter 750 (FIG. 7) as specifiedin document 610 (FIG. 7), client computer 210 traverses cache table 235,searching for a matching OID value in column 247. As can be seen fromFIG. 8, a match is found in row 238-2. Client computer 210 thenreferences the corresponding cache pointer column 249 for row 238-2,which points to cache element 520 within cache 230. Client computer 210then copies cache element 520 from cache 230 to its own memory as shownby arrow 840. It will be noted that the copy loaded, as shown by object810, is equal to the contents previously downloaded as shown in FIG. 5from server SYSA 215, and is not freshly loaded from server SYSB 615(FIG. 6). That is, the cache copy was obtained without regard for thelocation from which it was originally obtained server SYSA 215 or serverSYSB 615, or from the location specified in HTML document 510.

Thus, FIGS. 2 through 8 depict a location-independent means of obtainingcopies of resources, which would otherwise require redundanttransmission of copies of resources identical to copies that had alreadybeen previously obtained. Although the examples shown depict retrievalof a data object in the form of a Java applet or a GIF file, theinvention may also be applied to retrieval of any machine-readableresource, for example, a portion of a computer program such as asubroutine or other program segment, a text file, a sound file, or adata object that comprises a collection of multiple data objects.

FIG. 9 is a flow chart depicting the overall operation of the invention.Execution begins in step 910. In step 920, the client computerdetermines what resource is desired to be obtained, e.g., by examiningan applet tag in a previously downloaded HTML file. In step 925, theclient computer checks to see whether an OID was specified for thedesired resource. If no OID was specified, execution proceeds with step930. In step 930, the client computer obtains a copy using the URI, forexample by issuing an HTTP GET request to the server corresponding tothe URI.

In step 935, the client computer verifies that an OID was encoded intothe retrieved copy. If so, control proceeds to step 940. In step 940,the client computer checks to see whether the OID is valid, for example,by verifying a digital signature. If so, control proceeds to step 950.In step 950, client computer 210 updates the OID status in the cache forthe file obtained. If either no OID is specified in the retrieved copyas determined in step 935, or an OID is specified but is not valid asdetermined in step 940, step 950 is skipped, and control proceeds tostep 990 where processing of the retrieved object is complete. As notedpreviously, the steps of verifying the OID from the copy of theretrieved object, and validating the OID against the received object,are optional and may be omitted in certain environments.

Referring again to step 925, if an OID was specified, control proceedsto step 970. In step 970, the client computer inspects the cache tableto determine whether an object with the desired object identifier haspreviously been downloaded and is available in the cache. If no suchobject is found in the cache, control proceeds to step 930, as if no OIDhad been specified. If the desired OID entry is found in the cachetable, however, control proceeds to step 975, in which the clientcomputer retrieves the cache copy, loads it into memory and proceeds toexecute using the copy retrieved from the cache. In any event, afterstep 975, processing for the object exits at step 990.

FIG. 10 depicts an alternate embodiment of the present invention. Asshown in FIG. 10, a plurality of clients 1010 and 1020 are incommunication with a firewall server 1040 using communication links1030. Firewall server 1040 may be, for example, a proxy server, afirewall or a gateway. Cache 1050 is operated under the control offirewall server 1040. In response to request from clients 1010 and 1020,firewall server 1040 makes requests to various servers in network 1060over communication link 1070.

In the embodiment depicted it is not necessary for clients 1010 and 1020to maintain their own caches. Instead, requests for resources areprovided to the firewall server 1040 and include both thelocation-dependent URI associated with the specific copy of theresource, as well as the location independent OID (if any) associatedwith the resource. Firewall server 1040 maintains cache 1050 aspreviously described. When an object with an OID corresponding to an OIDrequested by one of clients 1010 or 1020 is found in cache 1050,firewall server 1040 responds to the respective client with a copy ofthe object as obtained from the cache. If no such object is found in thecache 1050, firewall server 1040 uses the supplied URI to obtain a newcopy from network 1060.

While the invention is described in terms of preferred embodiments in aspecific system environment, those skilled in the art will recognizethat the invention can be practiced, with modification, in other anddifferent hardware and software environments within the spirit and scopeof the appended claims.

Having thus described our invention, what we claim as new, and desire tosecure by Letters Patent is:
 1. A method for obtaining a copy of a dataobject, comprising the steps of:a) obtaining a location-independentidentifier for the data object from a primary file; b) interrogating acache using the location-independent identifier for the data object todetermine whether the data object is cached; and c) if the data objectis cached, obtaining the copy of the data object.
 2. The method asrecited in claim 1, further comprising the step of:d) if the data objectis not cached, performing a network call to obtain the copy of the dataobject.
 3. The method as recited in claim 1, in which steps (b) and (c)are performed by a client program.
 4. The method as recited in claim 1,in which the location-independent identifier is an ASN.1 objectidentifier.
 5. The method as recited in claim 1, in which the dataobject comprises logic and data.
 6. The method as recited in claim 1, inwhich the data object is a computer program.
 7. The method as recited inclaim 1, in which the data object is a class object.
 8. The method asrecited in claim 1, in which the data object is a program subroutine. 9.The method as recited in claim 1 in which the data object is an imagefile.
 10. The method as recited in claim 1 in which the data object is atext file.
 11. The method as recited in claim 1 in which the data objectis a multimedia file.
 12. The method as recited in claim 1, in which thedata object comprises an executable program segment.
 13. The method asrecited in claim 12, in which the executable program segment is in aplatform-independent object code form.
 14. An apparatus for obtaining acopy of a data object, the apparatus comprising:(a) a computer and acache; (b) the cache being indexed by a cache table; and (c) thecomputer being responsive to requests for the data object having alocation-independent identifier, whereby the computer interrogates thecache using the location-independent identifier for the data object todetermine whether the data object is cached, and if the data object iscached, obtains the copy of the data object from the cache.
 15. Theapparatus as recited in claim 14, whereby if the data object is notcached, the computer performs a network call to obtain the copy of thedata object.
 16. The apparatus as recited in claim 14, in which thelocation-independent identifier is an ASN.1 object identifier.
 17. Theapparatus as recited in claim 14, in which the data object comprises anexecutable program segment.
 18. The apparatus as recited in claim 17, inwhich the executable program segment is in a platform-independent objectcode form.
 19. The apparatus as recited in claim 14, in which the dataobject is a computer program.
 20. The apparatus as recited in claim 14,in which the data object is a class object.
 21. The apparatus as recitedin claim 14, in which the data object is a program subroutine.
 22. Theapparatus as recited in claim 14, in which the data object is an imagefile.
 23. The apparatus as recited in claim 14, in which the data objectis a text file.
 24. The apparatus as recited in claim 14, in which thedata object is a sound file.
 25. A system for the transmission of a dataobject, the system comprising:(a) a client computer having a cache, thecache being indexed by a cache table; (b) at least one server computer;(c) the client computer and the server computer coupled by acommunications link; and (d) the client computer being responsive torequests for the data object having a location-independent identifier,whereby the client computer interrogates the cache using thelocation-independent identifier for the data object to determine whetherthe data object is cached, and if the data object is cached, obtains acopy of the data object from the cache, and otherwise calls the servercomputer to obtain the copy of the data object.
 26. The system asrecited in claim 25, in which the location-independent identifier is anASN.1 object identifier.
 27. The system as recited in claim 25, in whichthe data object comprises an executable program segment.
 28. The systemas recited in claim 27, in which the executable program segment is in aplatform-independent object code form.
 29. The system as recited inclaim 25, in which the data object is a computer program.
 30. The systemas recited in claim 25, in which the data object is a class object. 31.The system as recited in claim 25, in which the data object is a programsubroutine.
 32. The system as recited in claim 25, in which the dataobject is an image file.
 33. The system as recited in claim 25, in whichthe data object is a text file.
 34. The system as recited in claim 25,in which the data object is a sound file.
 35. A computer programembodied on a computer-readable medium for obtaining a copy of a dataobject, comprising:(a) a first code segment for obtaining alocation-independent identifier for the data object from a primary file;(b) a second code segment for interrogating a cache using thelocation-independent identifier for the data object to determine whetherthe data object is cached; and (c) a third code segment for, if the dataobject is cached, obtaining the copy of the data object.
 36. Thecomputer program as recited in claim 35, further comprising:d) a fourthcode segment for, if the data object is not cached, performing a networkcall to obtain the copy of the data object.
 37. The computer program asrecited in claim 35, in which the location-independent identifier is anASN.1 object identifier.
 38. The computer program as recited in claim35, in which the data object comprises an executable program segment.39. The computer program as recited in claim 36, in which the executableprogram segment is in a platform-independent object code form.
 40. Thecomputer program as recited in claim 35, in which the data object is acomputer program.
 41. The computer program as recited in claim 35, inwhich the data object is a class object.
 42. The computer program asrecited in claim 35, in which the data object is a program subroutine.43. The computer program as recited in claim 35, in which the dataobject is an image file.
 44. The computer program as recited in claim35, in which the data object is a text file.
 45. The computer program asrecited in claim 35, in which the data object is a sound file.